Processes for the disproportionation and transalkylation of toluene and heavy aromatics are important ways to produce aromatics of eight carbon atoms (referred to as C8A hereinafter). Up to date, as the typical and well-established process for this purpose, Tatoray process was developed in the late 1960s, MTDP process in the late 1980s, TransPlus and S-TDT processes recently.
In Tatoray process, toluene and aromatics of nine carbon atoms (referred to as C9A hereinafter) are used as the feed materials, with the level of heavy aromatics of ten and more carbon atoms (referred to as C10+ hereinafter) strictly controlled in the feedstock. In order to improve economic benefit and lower energy and material consumption, continued effort has been made to improve the Tatoray process, with focus on the catalyst. As a result, the overall performances of the catalyst, including space velocity, operation cycle, etc are improved. Meanwhile, the latitude in selecting the aromatics feedstock is also improved; for example, the aromatics feedstock with increased average molecular weight can be employed. The feedstock containing increased level of heavy aromatics facilitates the formation of C8A; however, it necessitates an increased reaction temperature to achieve a certain conversion thereof. In that case, the yield of hydrocarbons of five and less carbon atoms (referred to as C5− hereinafter) increases and the yield of the product of interest decreases correspondingly. In addition, owing to high activity of the employed catalyst, hydrodealkylation side reaction of aromatics is also accelerated, and the benzene (referred to as Ben hereinafter) yield increases. Accordingly, the C8A yield and C8A/Ben ratio of the product decrease, adversely affecting the overall economic benefit of the combined aromatics plant. In a combined aromatics plant, a unit of the disproportionation and transalkylation is indispensable for it produces C8A. In view of this, an increased Ben or a decreased C8A yield obviously has a negative impact on the overall economic benefit of the combined aromatics plant. Besides, the increased level of heavy aromatics in the feedstock exacerbates coking on the catalyst, and shortens the operation cycle of this unit. In practice, the unit of the disproportionation and transalkylation cannot consume all the heavy aromatics produced in the combined aromatics plant. It follows that a large amount of heavy aromatics is not utilized in and inevitably discharged from the plant.
U.S. Pat. No. 4,341,914 disclosed a process of the disproportionation and transalkylation as shown in FIG. 1. In FIG. 1, the reference numeral 1 stands for xylene column I; 2 for heavy aromatics column; 3 for reaction zone; 4 for benzene column; 5 for toluene column; 6 for xylene column 11; 7 for C9A stream; 8 for C8+A feed stream; 9 and 10 for toluene; 11 for benzene; 12 and 13 for C8A; 17 and 19 for streams enriched in C10+; and 18 for stream enriched in C9A. In this process, a portion of the resulting aromatics of ten carbon atoms (referred to as C10A hereinafter) is recycled to reaction zone along with the recycled C9A (stream 18), so as to inhibit production of C10+. The C10+ originally present in the C8+A feed material, however, cannot be utilized; meanwhile, a portion of C9A originally present in the C8+A feed material is discharged along with C10+ out of the plant from the bottom of heavy aromatics column (stream 19). On account of the nature of the employed catalyst, the latitude in selecting the feed materials is limited: it is required that the indan level in C9A stream (stream 7) be lower than 1 wt %.
Chinese Patent No. 98110859.8 disclosed a process of the disproportionation and transalkylation as shown in FIG. 2. In this figure, the reference numeral 1 stands for xylene column I; 2 for heavy aromatics column; 3 for reaction zone; 4 for benzene column; 5 for toluene column; 6 for xylene column II; 7 for o-xylene column; 8 for C8+A raw material; 12 and 13 for C8A; 9 for toluene; 14 for stream enriched in C9A; 15 for the heavy hydrocarbons of eleven and more carbon atoms (referred to as C11+ hydrocarbons hereinafter); 16 for recycled toluene; 17 for benzene; 19 for o-xylene; and 20 for C9+A with or without o-xylene. This process is free from many of the disadvantages of the prior art in that it allows use of raw materials containing increased level of indane and C10A, and achieves increased conversion of C10+ heavy aromatics. The process, however, suffers such disadvantages as shortened catalyst service life, leading to a mismatch with the operation life of the plant, which is becoming longer and longer. In addition, the process achieves little in improving C8A yield.
To date, the prior art has been seeking to improve the existing processes for the disproportionation and transalkylation of toluene and heavy aromatics by modifying the catalyst in terms of one or more aspects or by altering the technical measures to isolate the reaction products. For example, attempt has been made to enhance the transalkylation capability of the catalyst with respect to heavy aromatics. It fails, however, to achieve a balanced results of ample latitude in selecting feed material of high level of heavy aromatics, high yield of C8A and low yield of light hydrocarbons.
The prior art exerts rigid control over the amount of C10+ hydrocarbons in the raw materials to increase the operation life of the catalyst. At present, however, it is possible for the raw material to include some C10+ hydrocarbons as a result of progress in catalyst technology. The raw material containing an amount of C10+ hydrocarbons increases yield of the product of interest under the conditions of increased conversion and higher space velocity. All the same, most of C10+ hydrocarbons still cannot be fully utilized.
Moreover, the patents mentioned above all focus on processing aromatic raw materials without regard for the role of the unit of the disproportionation and transalkylation, which produces C8A in a combined aromatics plant. In other words, the above patents fail to pay sufficient attention to the economic benefit of the reaction products, represented by the ratio of C8A/Ben of the products. Increased C8A yield will immediately raise the pX yield of the combined aromatics plant and better economic benefits will be achieved.
Thus, in the field of the disproportionation and transalkylation of toluene and heavy aromatics, there exists a need for a new process which is free from the disadvantages of the prior art, i.e. low utilization of heavy aromatics in a combined aromatic plant, a limited C8A yield and a rigid control over the selection of the raw materials. By employing such a process, ample latitude in selecting materials, a decreased formation of light hydrocarbons, full utilization of heavy aromatics and increased C8A yield are achieved.